The West is blaming President Bashar al Assad's regime for chlorine attacks in Syria, but his ally Russia says it needs more proof.

The UN Security Council approved a resolution Friday condemning the use of chlorine as a weapon in Syria, amid Western accusations that President Bashar al Assad's government continues to include the chemical in barrel bombs.

Russia, one of Assad's principal backers in the Council, agreed to the text, but made clear it doesn't believe Assad was responsible for the incidents, or for a chemical gas attack launched in a Damascus suburb in August 2013. Venezuela was the sole abstaining vote.

"The use of chemical weapons presents a clear threat to international peace and security," United Kingdom Ambassador Mark Lyal Grant told the Council. "Today's important resolution puts the Syrian regime on notice that if we receive further credible reports of use of chlorine as a weapon, then this Council will take action."

Chlorine, commonly used to treat water and clean clothing, is not in itself a banned substance. Instead, it is considered a "dual-use" chemical under the 1997 Chemical Weapons Convention (CWC), which Syria signed in October 2013. However its deployment as a weapon is strictly prohibited.

The text threatens that any actors who use chemical weapons, including chlorine, should be held accountable and subject to measures under Chapter VII of the United Nations Charter. These could theoretically include the military force or economic sanctions, but either would require a further resolution — something Russia would be expected to veto.

Still, at a time of intense focus on the atrocities of Islamic State (IS) militants, the resolution recalls attention to alleged war crimes, including the use of chemical weapons, believed to be committed by the government in Damascus.

In December, American officials told a meeting of the Organization for the Prohibition of Chemical Weapons (OPCW) at The Hague that they believe Assad's regime has regularly deployed chlorine gas in crude barrel bombs dropped on civilian areas.

"There is compelling evidence that Syria continues to use chemical weapons systematically and repeatedly," US Under Secretary of State Rose Gottemoeller said.

A May 2014 Human Rights Watch (HRW) report concluded, based on witness interviews as well as video and picture analysis, that Assad's forces used bombs containing chlorine gas on three Syrian villages where armed rebel groups had been clashing with Syrian President Bashar al-Assad's troops. HRW noted that fragments of chlorine canisters found at the scene indicated that they had been dropped from considerable heights, and strongly suggesting that they'd been deployed as crude "barrel bombs" containing chlorine canisters from Syrian government helicopters. Opposition factions are not known to have access to aircraft.

The OPCW said in September 2014 it had "compelling confirmation" that a toxic chemical, likely pure or mixed chlorine, had been used "systematically and repeatedly" in the same part of the country. Chlorine was used in attacks on three villages, the mission concluded, after conducting dozens of interviews with victims, doctors, and eyewitnesses, as well as reviewing medical records, video, and other documentation.

"The descriptions, physical properties, behavior of the gas, and signs and symptoms resulting from exposure, as well as the response of patients to the treatment, leads the FFM [fact finding mission] to conclude with a high degree of confidence that chlorine, either pure or in mixture, is the toxic chemical in question," the OPCW said.

At the Friday Council session, Russian Ambassador Vitaly Churkin said the resolution should not be seen as an indictment of the Syrian regime, which, he claimed, hadn't been conclusively tied to any chemical weapons attacks. He also called into question the veracity of OPCW reporting.

"We categorically do not accept the possible use of sanctions under Chapter VII without an attempt to confirm this based on proof," he told the Council. Both Churkin and Syrian ambassador Bashar Jaafari — who spoke to reporters after the vote — maintained that chlorine had in fact been used by "terrorists."

American Ambassador Samantha Power said the accounts of helicopters dropping barrel bombs filled with chemical weapons was conclusive.

"Who has helicopters in Syria? Certainly not the opposition. Only the regime does," she said.

Chlorine gas was first used in battle 100 years ago, during a 1915 World War I battle in Ypres, Belgium. Exposure can cause burns and, if inhaled, respiratory failure and vascular abnormalities. High-level exposure can kill in minutes.

The OPCW has been overseeing the disassembling and destruction of Syria's chemical weapons stockpile after Assad's government signed the CWC in October 2013, following a chemical assault on a Damascus suburb in August that year that killed as many as 1,400 people. HRW subsequently concluded that evidence "strongly suggests" Syrian government forces were responsible for the Damascus attack, which was apparently conduced with "a weapons-grade nerve agent" likely to be sarin gas. In 2012, US President Barack Obama had previously suggested that Syrian chemical weapon use would prompt American action against Assad.

Churkin made light of Obama's "red line," saying the lack of subsequent American intervention proved the Syrian regime had done nothing wrong.

Assad has repeatedly denied that his military was using chlorine or any other form of chemical weapon. In a November 2014 interview with Paris Match he said his forces had more effective munitions at their disposal.

"You can find chlorine in any house in Syria," he said. "Everyone has chlorine, and any group can use it. But we haven't used it because we have traditional weapons, which are more effective than chlorine, and we do not need to use it. We are fighting terrorists, and using traditional weapons without concealing that or being shy about it. So, we don't need chlorine."

He went on to say that Syrian government forces couldn't possibly have used chemical weapons, because if they had, the death toll would have been much higher.

"We haven't used this kind of weapons; and had we used it anywhere, tens, if not hundreds, of thousands of people would have died," Assad said. "It's impossible for these weapons to kill, as it was claimed last year, only one hundred people or two hundred people, particularly in areas where hundreds of thousands, and maybe millions, of Syrians live."

He made similar claims in an interview with the BBC earlier this year.